One of the most important considerations in designing an optoelectronic system is the connection between circuit components such as integrated circuits or optoelectronic devices with purely optical components such as waveguides. Optoelectronic devices such as lasers need be optically coupled to fibers to engage in optical fiber communications. High performance optoelectronic devices are generally assembled in optical packages, e.g., butterfly packages, including a housing. These package housings have an aperture in one of their walls from which the optical beam generated by the laser emerges. The beam propagates through free space within the package and exits from the package aperture to couple to an optical fibre. In telecommunications, the beam generated by a laser device, typically a semiconductor laser diode, needs to be coupled to a transmission optical fibre, which for most applications in telecommunications is a single-mode optical fibre.
External-cavity laser systems including a gain medium (typically a semiconductor laser diode) and optical components that collimate the emitted optical beam and select the wavelength to be transmitted are advantageously employed as transmitters in WDM systems.
U.S. Pat. No. 6,526,071 describes an external-cavity tunable laser that can be utilized in telecom applications to generate the centre wavelengths for any channel on the International Telecommunications Union (ITU) grid. The disclosed tunable laser includes a gain medium, a grid generator and a channel selector, both grid generator and channel selector being located in the optical path of the beam. The grid generator selects periodic longitudinal modes of the cavity at intervals corresponding to the channel spacing and rejects neighbouring modes. The channel selector selects a channel within the wavelength grid and rejects other channels. The grid generator is dimensioned to have a free spectral range (FSR) corresponding to the spacing between gridlines of a selected wavelength grid (an ITU grid) and the channel selector is dimensioned to have a FSR broader than that of the grid generator which is itself broader than the FSR of the cavity.
An external cavity tunable laser with a FP etalon as grid generator and an LC-based tunable mirror is described in WO patent application No. 2005/041371. Active alignment is usually employed to align the fibre with respect to the laser. After the fibre is actively aligned to the laser, the alignment process is completed by mounting its ferrule or other fibre housing to a laser housing, such as a TO-can package, by a variety of techniques such as laser welding, or by using an appropriate adhesive such as epoxy or glue. In the coupling of the laser beam to a transmission fibre, it is desired to optimize the power transfer efficiency and component alignment tolerances.
The coupling between a laser device and a fibre can be realized inside of the package in which the laser is contained (i.e. the fibre is located directly in front of the chip). Therefore, in order to make this connection, some free space has to be left in the package and additional optical elements are difficult to place there between.
The connection between the laser and the fibre can also be realized by using a connecting assembly known as “collimator” which has to be aligned to the collimated laser beam path and then fixed to the packaging in which the laser is contained. The collimator is designed to focus a collimated laser beam to an end of an optical fibre. Typically, a collimator is a pre-assembled assembly including an optical fibre termination, with a fibre end, and a lens that is positioned and fixed relative to the fibre termination prior to attachment to the device package, so that an optical beam of approximately constant cross-section, i.e., a collimated beam, can be coupled to or from the fibre. Generally, the lens and the optical fibre termination are fixed inside a cylindrical metallic sleeve. Additional optional optical elements can also be present in the collimator. Alternatively, the different components included in the collimator may be assembled using laser welding when the collimator is aligned in front of the package. This technique, which requires a collimated beam emerging from the package, includes the step of positioning a focussing lens in front of the laser beam through a step of active alignment, so that, the optical axis of the beam exiting the package lies substantially perpendicular, e.g., within 0.5° from the perpendicular axis, to the package wall containing the aperture from which the beam emerges.
The fibre is then to be assembled: a sleeve in which a ferrule is contained is actively aligned to the holder and then the ferrule is fixed, optimizing the position of the fibre with respect to the focal point of the lens. Post-welding alignment may also be necessary. Alternatively, a simultaneous alignment of all elements (sleeve, holder and ferrule) may be possible, provided that a suitable workstation is available. Workstations especially designed to fabricate assemblies including a package and a collimator using the above technique are sold for example by ELS Elektronik Laser System GmbH or by Moritex USA Inc, the respective products' names being “Versaweld” and “FZ-80”.
The U.S. Pat. No. 6,702,476 in the name of Agere Systems Inc. and Triquint Technology Holding Co. describes a fibre optic device and a method of assembly the fibre optic device to provide at least four degrees of adjustment between an optical fibre and bulk optics in an optical device package. At an opening in the wall of the optical processor package, a washer-like structure allows initially adjustable contact at a flat surface thereof and at a spherical curved surface thereof. One of the surfaces bears against the wall about the opening and the other surface bears against a contact feature of a connecting assembly, typically a pre-assembled collimator assembly, which holds a termination portion of the optical fibre. Translational adjustment is made at the flat surface and tilting adjusting is made at the curved surface before final attachment at both surfaces. The fibre optic device is thus aligned with the optical device package and, through that package, can be aligned with another connecting assembly that is initially fixed in place at another opening of the optical device package. Circumferential tapers are provided at the appropriate ones of the surfaces, the wall, and the contact feature to facilitate adhesive flow for final attachment. After adhesive placement, it is UV-tacked and heat curing is subsequently performed.
Japanese patent application No. 09-015447 in the name of Oki Electric Ind. Co. LTD discloses a structure and method for fixing optical fibre collimator, so that stable characteristic is kept. An annular fixing and coupling member having a through-hole in the centre is allowed to intervene between the optical fibre collimator constituted by attaching an optical fibre and an optical lens in a sleeve so that their optical axes may be aligned and the optical functional part so as to be nearly aligned with the optical axis of the optical functional part, and a spherical surface and a spherical receiving part are positioned between the collimator and the member, and the spherical surface and an edge part are allowed to abut and coupled and fixed by welding.
Japanese patent application No. 2004-077985 in the name of Japan Aviation Electronics Industry Ltd shows an optical module for aligning optical element e.g. light modulator, optical fibre, adjusts projection angle of light which irradiates optical fibre holder and optical element. The spherical surfaces of optical fibre aligning saddle and fibre aligning welding saddle acts as an adjustment unit. The adjustment unit adjusts the projection angle of light which irradiates an optical fibre holder and an optical element e.g. a light modulator.